The invention relates to a micromechanical device and a method for its production.
Micromechanical devices such as sensors or actuators are increasingly finding their way into all areas of technology, for example into navigation systems and motor vehicles, particularly in connection with safety systems. Pressure and acceleration sensors form a large proportion of devices of this type. Sensors are required which are reliable, small, easy to produce and at the same time inexpensive and have a high measuring accuracy and good proportionality between the measured value and the output signal. The same is correspondingly true for actuators, so that, for the sake of simplicity, only sensors will be discussed below.
The majority of pressure or acceleration sensors used nowadays are produced by precision mechanics or by means of a KOH etching technique based on silicon (bulk micromachining). The sensor signal, generated to date mainly by means of the piezoelectric effect, is evaluated separately from the sensor. However, the trend is toward the intelligent sensor, in which the sensor and the circuit for evaluating the sensor signal and, if appropriate, a test circuit are integrated on a chip on the basis of silicon planar technology. The evaluation of the piezoresistive or capacitive sensor signal and the linearization and amplification take place using semiconductor circuits of known technologies. A sensor of this type is disclosed, for example, in the publication F. Goodenough: Airbags Boom When IC Accelerator Sees 50 G, Electronic Design, Aug. 8, 1991, pp. 45-56.
Whereas conventionally produced micromechanical sensors are relatively large, expensive and inaccurate, the above-mentioned publication describes an improved embodiment. This known, so-called surface-micromechanical sensor (surface micromachining) requires, as emerges, in particular, from the further publication relating to this: Analog Devices Combine Micromachining and BICMOS, Semiconductor International, Oct. 1991, 21 masks for its production, namely 6 masks for the sensor process and 15 masks for a 4 .mu.m BICMOS process. The comb-shaped sensor element for forming the capacitive sensor comprises a polysilicon element 2 .mu.m thick and is connected to the substrate surface by means of springs, which are likewise made of polysilicon.
A further capacitive structure is disclosed in U.S. Pat. No. 5,025,346.
The production method for the known sensors and actuators is extremely complicated and expensive. Furthermore, it is uncertain whether the polysilicon layers used for the mechanically moving parts of a sensor have adequate mechanical long-term stability. In addition to this possible degradation over time, the mechanical properties such as the modulus of elasticity or intrinsic stress of polysilicon are sensitively dependent on the respective process conditions during production. Thermal annealing of the intrinsic stress requires additional tempering steps in the production process, which has a disadvantageous effect on the electronic circuit which is simultaneously integrated in the sensor. Additional depositions of semiconductor layers are also necessary in the production process. In the case of a conceivable use of modern sub-.mu.m BICMOS circuits for the evaluation circuit of the sensor, it is no longer possible to produce stress-free polysilicon layers on account of the low process temperatures used in this case.
One problem relates to the processing of micromechanical, if appropriate integrated micromechanical, devices which are produced on a semiconductor wafer. In order to separate the chips, the wafer is ground thin and the individual chips are subsequently sawn. In this case, the filigree structure of the micromechanical device must be covered with a film on the front side of said device. A clean room is required for processing in order that particles which might impair the usability of the micromechanical device or render it unusable cannot get into said micromechanical device. This processing procedure is expensive and not very practicable even with large batch quantities.
A micromechanical device which has been separated as a chip must be fitted into a housing for protection against external influences. A plastic housing is ruled out for known devices, since the mobility of the sensor is lost when the chip is sheathed in plastic. Molding pressures up to 80 bar can lead to the complete destruction of the micromechanical device. A micromechanical chip is therefore usually mounted in a cavity-type housing, which is, however, about 10 times more expensive than a plastic housing.
The publications K. Ikeda et al.: Silicon pressure sensor with resonant strain gauge built into diaphragm, Proc. of the 7th Sensor Symp., Tokyo, Japan, 1988, pp. 55-58 and K. Ikeda et al.: Three-dimensional micromachining of silicon pressure sensor integrating resonant strain gauge on diaphragm, Sensors and Actuators, A21-A23, 1990, pp. 1007-1010 disclose micromechanical pressure sensors having a polysilicon diaphragm on which a resonator which reacts to mechanical stresses is arranged. When the diaphragm bends, the resonant frequency of the resonator changes on account of the mechanical stresses. In order that the externally exerted pressure cannot act directly on the resonator and thereby lead to measurement signal corruption, the resonator on the diagram is covered with a cap. A sensor of this type must also be mounted in a cavity-type housing for protection against environmental influences.
European reference EP-A 0 451 992 describes a micromechanical device and a corresponding production method, which device has a movable element made of polysilicon and situated in a cavity. The cavity is closed by an oxide layer.
The invention has the object of specifying a micromechanical device which can be produced more simply and more cost-effectively, and a method for its production.